When mendeleev started his work on organizing the elements, about how many had
been discovered?
about 10
well over 100
about 60
the same as today

About 60.

Explanation:

Mendeleev knew of 63 elements. He wrote their properties on cards and arranged them in order of atomic mass.

That's how he discovered that the properties of elements are periodic functions of their atomic masses.

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.

the atomic mass is the bottom number on the element.

-steel jelly